1. Field of the Invention
The present invention relates to a thin film transistor and an active array substrate having the same, and more particularly to a bottom gate thin film transistor and an active array substrate having the same.
2. Description of Related Art
The increasing progresses of the display technology bring great conveniences to people's daily life, in which flat panel displays (FPDs) become popular due to its feature of being light and thin. In various FPDs, liquid crystal displays (LCDs) are widely used due to its advantages of high space utility rate, low power consumption, free of radiation, and low electromagnetic interference, and the like.
In general, the LCD is mainly assembled by an active array substrate, a color filter substrate, and a liquid crystal (LC) layer sandwiched between the two substrates. The active array substrate has an active region and a peripheral circuit region. An active array is located within the active region and a driving circuit including a plurality of bottom gate thin film transistors (TFTs) is located within the peripheral circuit region.
Generally, for the consideration of high mobility, high stability, and low cost, semiconductor layers of the bottom gate TFTs in the driving circuit are manufactured with amorphous. A channel current Ion of the TFT is proportional to a ratio of the width and the length (W/L) of the channel and satisfies the following equation:
Ion=Cox*U*W/L(VG−Vth)VD, wherein Cox is the gate capacitance per unit area, U is mobility of carriers, W is channel width, L is channel length, VG is gate voltage, Vth is threshold voltage, and VD is drain voltage. Consequently, the channel current Ion is increased by increasing the channel width. However, the increase in channel width usually affects the layout of bottom gate TFTs.
Conventionally, multiple pairs of sources and drains are disposed to avoid the effect on the device layout. Moreover, the ratio of the channel width and length is increased by arranging the sources and the drains alternately. Nevertheless, even though this method increases the channel current, but the self-heating resulted from the high channel current can not be conducted out effectively, so as to result in poor reliability of the devices.